cmd/upspin: allow share -fix to remove users
cmd/upspin: allow share -fix to remove users

report the Keys: user as "unknown" and remove him from the readers.

Fixes #348

Change-Id: I0021f91611a69c19a0e94e285d11f577138ce2d3
Reviewed-on: https://upspin-review.googlesource.com/8675
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin